This invention relates to actuating fuel trim valves in a gas turbine applied to tune each combustion chamber. In particular, the invention relates to a system and method for automatically actuating fuel trim valves to a plurality of combustion chambers.
Combustors in industrial gas turbines have a plurality of combustion chambers arranged around a turbine casing. High pressure air from the compressor flows into the chambers where the air is mixed with fuel. Fuel is injected into the chambers through nozzles. Hot gases generated by the combustion of the air and fuel mixture flow from the combustion chambers into the turbines which generally include a high-pressure turbine to drive the compressor and a low-pressure turbine to provide output power.
Each combustion chamber defines a generally cylindrical combustion zone. Upstream of the combustion zone, the chambers each have a plurality of fuel nozzles that inject fuel into the zone. Fuel flow to each nozzle (or group of nozzles) is regulated by a trim valve. Adjusting the trim valve provides a degree of precise control of the amount of fuel flowing to each fuel nozzle in each combustion chamber. Trim valves may be used to tune fuel flow to each combustion chamber in a gas turbine such that combustor pressure oscillations, nitrous oxides, carbon monoxide, and unburned hydrocarbons are minimized. A prior fuel trim system is disclosed in published U.S. patent application Ser. No. 2003/0144787 A1.
Fuel trim valves are commonly used to adjust the fuel entering each nozzle of a combustion chamber in a multi-chamber combustor of an industrial gas turbine. Generally, trim valves are used to optimize the mixture of fuel and combustion air entering each combustion chamber such that the combustion of the air-fuel mixture minimizes the production of nitrous oxides (NOx), carbon monoxide (CO) and unburned hydrocarbons (UHC). To minimize CO and UHC and achieve overall greater efficiency, it is desirable to increase the combustion temperature within the gas turbine. However, the oxidation of NOx in gas turbines increases dramatically with the increase in combustion temperatures.
Fuel trim valves provide a means to adjust the fuel flow to individual nozzles and combustion chambers to compensate for the variations in the fuel-to-air ratio to each chamber. Setting the air-fuel ratio often involves a careful balance between: (1) increasing gas turbine efficiency and/or minimizing unburned hydrocarbons carbon monoxide (UHC) and carbon monoxide (CO) by increasing combustion temperature and (2) decreasing the combustion temperature to minimize nitric oxides (NOx) by thinning the air-fuel ratio. It is extraordinarily difficult to achieve uniform temperature and pressure distributions in the multiple combustion chambers of an industrial gas turbine. Variations in the airflow between the combustion chambers make it difficult to maintain a constant air-fuel ratio in all combustion chambers. CO emissions tend to be more sensitive to fuel-to-air ratio variations from chamber to chamber than are NOx emissions. Tuning airflow to individual combustion chambers may be applied to reduce the overall level of CO emissions while maintaining satisfactory gas turbine operation.
Fuel supply systems are known that have multiple manifolds for supplying fuel nozzles with fuel in each combustion chamber of a multi-chamber gas turbine. It is not conventional to have a fuel trim valve for controlling each fuel nozzle in each combustion chamber or to provide each fuel nozzle in each combustion chamber with a fuel trim valve. There are several difficulties to providing fuel trim valves to each fuel nozzle in each combustion chamber including: (1) there is limited piping room in a gas turbine engine to incorporate a fuel trim valve for each fuel nozzle; (2) to increase efficiency it is typical to incorporate multiple fuel manifolds so that the pressure drop across each fuel trim valve is within a small uniform range; and (3) manually adjusting each of the fuel trim valves in each combustion chamber is a Herculean task.
There is a long-felt need for systems and methods to control the air-fuel ratio of a multi-chamber gas turbine. What is also needed are systems and methods to control the air-fuel mixture in each combustion chamber of a multi-chamber gas turbine such that the combustion chamber pressure oscillations are reduced, and emissions of combustion chamber pressure oscillations, NOx, UHC and CO are minimized for a given energy output level of the gas turbine. There is also a need for simple systems and methods for automatically adjusting each fuel valve in each combustion chamber, such that the air-fuel ratio in each combustion chamber can be optimized to minimize combustion chamber pressure oscillations and reduce emissions of NOx, UHC and CO for the gas turbine. There is also a need to devise a trim valve tuning strategy that interprets the gas turbine combustion dynamics, emissions, exhaust temperature spreads and other sensor data so as to achieve a proper setting for each fuel valve that trims flow for each combustion chamber.